This invention relates to an adhesive composition and methods for using an adhesive composition in die packaging applications.
In semiconductor manufacture, a single semiconductor die (or chip) is typically mounted within a sealed package. In general, a typical package protects the die from damage (e.g., breakage, physical abuse, etc.) and from contaminants in the surrounding environment (e.g., moisture, dust particles, etc.). In addition, the package provides a lead system for connecting electrical devices of the die to a printed circuit board or other external circuitry.
Each die has a lower surface (also referred to as the back of the die) that is devoid of circuitry, and an upper surface (also referred to as the face of the die) having integrated circuitry constructed thereon. The integrated circuitry is electrically accessible via die wire bonding pads which may be arranged in a variety of configurations on the face or edges of the die.
Typically, an initial component in the packaging process is a leadframe. The leadframe is a metal frame which supports several dice for packaging and provides the leads for a final semiconductor package. A typical leadframe strip is adapted to mount several semiconductor dice.
In one conventional packaging process, each die is attached to a mounting paddle of the leadframe utilizing an adhesive layer. The adhesive layer is typically formed from an adhesive composition including an epoxy, an acrylic, a silicone or a polyimide material located between the bottom surface of the die and an upper surface of the mounting paddle. Also, during the packaging process, bond pads formed on the die are electrically connected to the leads of the leadframe using bond wires. Following the application of a protective layer, for example polyimide, to the face of the die, it and a portion of the leadframe to which it is adhered is encapsulated in a polymeric material. Similarly, all other die-leadframe assemblies on a leadframe strip are encapsulated resulting in encapsulated interconnected packages, i.e., interconnected by way of the leadframe. After encapsulation, a trim and form operation separates the interconnected packages. Leads of each package are also shaped into a desired configuration.
Attempts have been made to increase circuit densities and to improve packaging technologies. For example, some high density circuits, such as 16 mega bit memory devices, use lead-on-chip (LOC) packaging technology. A known lead-on-chip (LOC) semiconductor package is described in U.S. Pat. No. 4,862,245 (Pashby et al.). In general, a LOC die package is formed with a leadframe that does not include a mounting paddle for mounting the die. In the LOC die package, lead fingers of the leadframe are not only for electrical attachment to the bond pads of the die via bond wires but also adhere to the face of the die and support the die during processing. Prior to encapsulation, the die, in effect, is mounted to the lower surface of lead fingers. This configuration provides improved heat transfer from the die and shortens the length of the bond wires. In addition, this configuration enhances the reliability of the package seal.
One method of attaching and wire bonding a LOC die to an LOC leadframe is known as area wire bonding or A-wire bonding. When A-wire bonding is automated, the method typically includes attaching a double-side adhesive tape to the bottom surface of the leadframe fingers. Typically, the double-sided adhesive tape includes a thermoset adhesive on two opposing surfaces of the tape. The face of the die is then attached to a tape surface opposite the leadframe fingers. Generally, attaching the face of the die to the tape surface is accomplished in the presence of heal and pressure. The die-tape-leadframe fingers construction is then heated in an oven to cure the thermoset adhesive. After the adhesive is cured, the leadframe is transferred to a wirebonder machine that connects the bond wires to the bond pads on the die and to the lead fingers of the lead frame. Holes though the adhesive tape allow the bond wires to attach to the bond pads on the die.
In general, an A-wire process is relatively expensive and complicated. For example, the double-sided adhesive tape used in the process is an expensive component because the tape must have an accurate configuration and must be precisely attached to the leadframe fingers for each die to be attached. This is a delicate process and requires precise indexing of the die and precise alignment of the adhesive tape with the die and the leadframe fingers. Moreover, any irregularities (such as buckles, gaps, wrinkles, etc.) in the adhesive tape during attachment to the leadframe fingers tends to cause voids and adhesion problems during attachment of the die, which may ultimately result in the production of a defective product.
In another packaging method, a lead-under-chip (LUC) die may be attached to leadfingers of a leadframe. For example, a tape adhesive on the back of the die or on the leadfingers, along with positioning the die on the lead fingers, results in attachment of the die to the leadframe.
Additionally, because the tape requires an accurate configuration primarily based on the die configuration, many tape configurations (which can vary in tape width, hole patterns, etc.) are required to be stocked by semiconductor manufacturers. This also adds to the expense and complexity of the process. Furthermore, when hole patterns are punched out, a large amount of tape is wasted in the process.
Yet another packaging method includes attaching a semiconductor die to a leadframe as described in U.S. Pat. No. 5,286,679 (Farnworth et al.). The method includes forming a patterned layer of a thermoplastic or thermoset adhesive to one surface of a semiconductor wafer. Individual dice are then singulated from the wafer. During packaging, each adhesive coated die is attached to lead fingers of a leadframe by heating the adhesive layer and pressing the lead fingers, adhesive and die together. This heating process is sufficient to cure the adhesive layer formed from a thermoplastic material. However, when the adhesive layer is formed from a thermosetting material, a separate heating step is required for curing.
Thus, whether adhesive tapes, thermoplastic adhesives or thermoset adhesives arm used, heat, typically at a temperature of about 300xc2x0 C. or more, is generally supplied to the leadframe-adhesive-die interface during packaging which adds cost, time and equipment to the packaging process.
In view of the foregoing, there is a need in the semiconductor art for an improved adhesive and methods for using such an adhesive in packaging applications. Accordingly, the present invention is directed to a method for forming a pattern of adhesive on a wafer, a method of attaching a semiconductor die to a leadframe, a method of making a semiconductor package, a semiconductor package and an adhesive composition suitable for use in packaging applications.
One aspect of the present invention provides a method for applying an adhesive to a wafer. Preferably, the method includes the steps of providing a wafer having a surface; and applying an instant setting adhesive composition on the surface of the wafer in a configuration wherein a plurality of portions of the surface have the instant setting adhesive composition applied thereon, and further wherein one or more zones of the surface are essentially free of the instant setting adhesive composition.
The method may also include the step of singulating the wafer to form at least one die having the instant setting adhesive composition on at least a portion thereof. The zones may include singulation streets or regions having exposed bond pads. Additionally, the method may further include the step of applying an adhesion promoter to the surface of the wafer prior to applying the instant setting adhesive composition Another aspect of the present invention provides a method for applying an adhesive to a wafer. Preferably, the method includes the steps of providing a wafer having a surface; and applying an instant setting adhesive composition on the surface of the wafer in a configuration wherein a plurality of portions of the surface have the instant setting adhesive composition dispensed thereon and one or more zones that are essentially free of the instant setting adhesive composition, wherein the instant setting adhesive composition has a thixotropic index from about 4 to about 6. Preferably, the one or more zones include singulation streets and regions having exposed bond pads and further comprising singulating the wafer along the singulation streets to form at least one die having the instant setting adhesive coated on at least a portion thereof.
According to the present invention, an instant setting adhesive composition is also provided. As utilized herein, the instant setting adhesive composition includes an adhesive component selected from the group of a cyanoacrylate adhesive, an anaerobic acrylic adhesive, and mixtures thereof; and at least one optional additive selected from the group of a thermal stabilizer, a thickener, a plasticizer, a toughener, a conductive filler, a dielectric additive, a moisture stabilizer, a curing inhibitor, an adhesion promoter, a storage stabilizer, a colorant, and an organic solvent. The instant setting adhesive composition preferably has a thixotropic index from about 4 to about 6. Additionally, the instant setting adhesive composition preferably has a thermal degradation temperature of about 300xc2x0 C. or more.
In one embodiment of an instant setting adhesive according to the present invention, the instant setting adhesive composition includes a cyanoacrylic adhesive component including a monomer of the formula: 
wherein R is selected from the group of a C1-6 alkyl, a cycloalkyl, an alkenyl, an alkynyl, a cycloalkenyl, an alkaryl, an aralkyl, and an aryl group. Preferably, R is selected from the group of a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, an isobutyl group, a pentyl group, a hexyl group, an allyl group, a methallyl group, a crotyl group, a propargyl group, a cyclohexyl group, a benzyl group, a phenyl group, a cresyl group, a 2-chlorobutyl group, a trifluoroethyl group, a 2-methoxyethyl group, a 3-methoxybutyl group and a 2oxyethyl group.
In another embodiment of an instant setting adhesive composition according to the present invention, the instant setting adhesive composition includes a cyanoacrylate adhesive component comprising a monomer of the formula: 
wherein R is selected from the group of a C1-6 alkyl, a cycloalkyl, an alkenyl, an alkynyl, a cycloalkenyl, an alkaryl, an aralkyl, and an aryl group; and at least one optional additive selected from the group of a thermal stabilizer, a thickener, a plasticizer, a toughener, a conductive filler, a dielectric additive, a moisture stabilizer, a curing inhibitor, an adhesion promoter, a storage stabilizer, a colorant, and an organic solvent. Preferably, the instant setting adhesive composition has a thixotropic index from about 4 to about 6. Additionally, the instant setting adhesive composition preferably has a thermal degradation temperature of about 300xc2x0 C. or more.
Yet another aspect of the present invention provides a method for use in packaging a die. Preferably, the method includes the steps of providing a die; providing a leadframe; and using an instant setting adhesive composition to attach the one or more dice to a portion of the leadframe. Preferably, the instant setting adhesive composition includes an adhesive component selected from the group of a cyanoacrylate adhesive, an anaerobic acrylic adhesive, and mixtures thereof and further wherein the instant setting adhesive composition has a thixotropic index from about 4 to about 6.
Preferably, the die includes the instant setting adhesive composition applied thereon. The instant setting adhesive composition can be applied on a variety of portions including at least a portion of the leadframe; a back surface of the die; and at least a portion of a face surface of the die, which can be exposed die bond pads.
Another aspect of the present invention provides a method for use in packaging a die. Preferably, the method includes the steps of providing a die; providing a leadframe; and using an instant setting adhesive composition to attach the one or more dice to a portion of the leadframe under pressure and a temperature of about 200xc2x0 C. or less.
The methods according to the present invention can further include a variety of additional steps. For example, the method can include the steps of using an instant setting adhesive composition comprising a conductive filler to form a heat sink; and attaching the heat sink to a portion of the die or the leadframe, preferably wherein the heat sink is attached to the package. The method can also include applying an encapsulant on portions of the die attached to the leadframe, wherein the encapsulant comprises an instant setting adhesive composition. Likewise, the method can further include using the instant setting adhesive composition comprises attaching a plurality of lead fingers of the leadframe to the die. Preferably, the lead fingers include the instant setting adhesive composition applied on at least a portion thereof. The method may also further include the step of using the instant setting adhesive composition comprises attaching the die to a mounting paddle. Preferably, the mounting paddle includes the instant setting adhesive composition applied on at least a portion thereof.
Additionally, the methods may further include the step of applying an encapsulant on portions of the die attached to the leadframe. Preferably, the encapsulant includes an instant setting adhesive composition. The method may also include the step of using the instant setting adhesive composition comprises attaching a plurality of lead fingers of the leadframe to the die, wherein the lead fingers include the instant setting adhesive composition applied on at least a portion thereof.
Yet another aspect of the present invention provides a method for attaching a semiconductor die to a leadframe. The method includes providing an instant setting adhesive composition including an adhesive component selected from the group of a cyanoacrylate adhesive, an anaerobic acrylic adhesive, and mixtures thereof and, preferably, applying the instant setting adhesive composition on at least a portion of a wafer including a plurality of dice. The method also includes the steps of singulating dice from the wafer, and attaching a die having the instant setting adhesive composition applied on at least a portion thereof to a portion of a leadframe. The method may also include the step of attaching the die on a portion of the leadframe. Preferably, attaching the die on a portion of the leadframe includes positioning a portion of the die having the instant setting adhesive composition thereon adjacent to the portion of the leadframe; and applying pressure at an elevated temperature to attach the die to the leadframe. More preferably, the elevated temperature is about 200xc2x0 C. or less.
A further aspect of the present invention provides a method for attaching a semiconductor die to a leadframe. Preferably, the method includes dispensing an instant setting adhesive composition on the leadframe, wherein the instant setting adhesive composition includes an adhesive component selected from the group of a cyanoacrylate adhesive, an anaerobic acrylic adhesive, and mixtures thereof. The method also includes placing the die in contact with the instant setting adhesive composition; and forming a bond between the die and the leadframe with the instant setting adhesive composition. The method may also include the additional step of applying a catalyst to the leadframe, die or to the instant setting adhesive composition prior to forming the bond between the die and the leadframe. In the method, the leadframe may be a mounting paddle or a lead-on-chip leadframe.
Yet a further aspect of the present invention provides a method for attaching a semiconductor die to a leadframe including the steps of providing the leadframe with a mounting paddle and dispensing an instant setting adhesive composition on the mounting paddle. The method also includes placing a die in contact with the instant setting adhesive composition; and applying pressure at a temperature of about 200xc2x0 C. or less to bond the die to the leadframe with the instant setting adhesive composition.
Another aspect of the present invention provides a method for attaching a lead-on-chip semiconductor die to a lead-on-chip leadframe. Preferably, the method includes providing the leadframe with a plurality of lead fingers configured to form a die mounting area and dispensing an instant setting adhesive composition on the lead fingers in the die mounting area, said adhesive material comprising an adhesive component selected from the group of a cyanoacrylate adhesive, an anaerobic acrylic adhesive, and mixtures thereof and an electrically insulating filler. The method also includes the steps of placing the die in contact with the instant setting adhesive composition; and forming a bond between the die and the lead fingers with the instant setting adhesive composition. Preferably, the method further includes the step of applying a catalyst to the lead fingers, die or the instant setting adhesive composition prior to the placing step.
In the methods according to the present invention above, the dispensing step preferably includes a method selected from the group consisting of screen printing, depositing and patterning, syringe applying, stenciling, dip coating, spraying, dot shooting, and combinations thereof. Alternatively, the step of dispensing the adhesive material includes forming a pattern of dots.
The present invention also provides a semiconductor package including one or more leads; a die including one or more die bond pads electrically connected to the one more leads; and an adhesive layer between at least a portion of the one or more leads and the die, the adhesive layer formed from an instant setting adhesive composition. The adhesive layer can be between a back side of the die and the at least one portion of the one or more leads or between a face side of the die and the at least one portion of the one or more leads. Preferably, the face of the die includes at least one bond pad connected to the one or more leads. Additionally, the semiconductor package may also include a heat sink attached to a portion of the die, wherein the heat sink is formed from an instant setting adhesive composition. The heat sink may be attached to a portion of the package.
Another aspect of the present invention provides a semiconductor package including a singulated portion of a leadframe including a mounting paddle and a plurality of trimmed lead fingers; a die; and an instant setting adhesive composition attaching the die to the mounting paddle.
Yet another aspect of the present invention provides a semiconductor package including one or more leads; a die including bond pads electrically connected to a portion of the one or more leads; and an adhesive layer between at least a portion of the one or more leads and the semiconductor die. Preferably, the adhesive layer is formed from an instant setting adhesive composition including a cyanoacrylate adhesive component comprising a monomer of the formula: 
wherein R is selected from the group of a C1-6 alkyl, a cycloalkyl, an alkenyl, an alkynyl, a cycloalkenyl, an alkaryl, an aralkyl, and an aryl group; and at least one optional additive selected from the group of a thermal stabilizer, a thickener, a plasticizer, a toughener, a conductive filler, a dielectric additive, a moisture stabilizer, a curing inhibitor, an adhesion promoter, a storage stabilizer, a colorant, and an organic solvent. Preferably, the instant setting adhesive composition has a thixotropic index from about 4 to about 6. Additionally, the instant setting adhesive composition preferably has a thermal degradation temperature of about 300xc2x0 C. or more.
The semiconductor packages in accordance with the present invention may also include an encapsulant formed from a composition comprising a component selected from the group of a cyanoacrylate adhesive, an anaerobic acrylate adhesive, and mixtures thereof.